Digital electronics have made it possible to record a grey scale or color image of a scene, as a still image, as a series of still images, or as a video. A video is a series of still images that continues for an extended period of time at a specific interval between each image. Analog imaging utilizes photographic film to obtain an image, whereas digital imaging utilizes a focal plane array (FPA) to obtain an image which provides a signal in response to light illumination that is then digitized. The FPA includes an array of light-detecting elements, or pixels, positioned at a focal plane of optics that image a scene. Much recent effort has been made to improve the density, size, sensitivity, dynamic range, and noise characteristics of FPAs, as well as the associated optics and electronics, enabling higher resolution images to be acquired.
The vast majority of imaging only acquires information in two dimensions, resulting in the flat images that are composed of an intensity value in a two-dimensional array. The position in the array is related to the position transverse to the direction the imaging system is pointing. Some imaging systems have added additional components that allow the measurement of the distance from the imaging system to the object(s) in the scene—that is, measurement of the “z-axis.” These 3D imaging systems provide an intensity and a distance for each pixel in the FPA. Many of these 3D imaging systems make use of a laser pulse that is transmitted by the imaging system to illuminate object(s) in the scene, and the system measures in some fashion the time required for the laser pulse to travel to the objects in the scene and return to the imaging system in order to measure the distance between the system and object(s) in the scene. As a class, these 3D imaging systems are generally referred to as time-of-flight 3D imaging systems.
Various techniques are used in current 3D imaging systems to make the distance measurement. For example, Advanced Scientific Concepts, Inc. of Santa Barbara, Calif. produces a system that uses an FPA where each pixel element is time-sensitive and is coupled with a high bandwidth read-out integrated circuit (ROIC) and a high bandwidth analog-to-digital converter (ADC) to generate a digital representation of the temporal profile of the returned light signal. Another technique is to modulate the output illumination from an array of LEDs, for example, and use a different type of time-sensitive FPA. Still another technique developed by the U.S. Air Force Laser Imaging and Ranging System (LIMARS) program uses an electro-optic modulator to produce an intensity value that is dependent on the timing of the returning light signal, referred to as a modulated imaging system (MIS). In all of these techniques, the length of the light pulse emitted by the 3D imaging system is short relative to the distance resolution desired. It is generally believed that using a longer light pulse or a slower modulation frequency to illuminate a scene in these systems will decrease the precision of the 3D imaging system. As used in this context, precision is related to the z-axis distance resolution that can be obtained by a 3D imaging system.
For high-precision distance measurement applications, it is necessary to obtain distance measurements with a resolution of millimeters or centimeters. Because the speed of light is typically around 300,000,000 m/s, a light pulse travels 1 mm in 3 ps and travels 1 cm in 30 ps. When current 3D imaging systems using light pulses of approximately 1-3 ns, that is equivalent to the light pulse having an extent of 30-90 cm. Thus, according to the conventional wisdom of those skilled in the art, these light pulses are too long to provide precise distance measurements in some applications requiring precision in the millimeter or low centimeter range. This is one of the principal reasons that current time-of-flight 3D imaging systems are limited to distance resolutions of greater than approximately 10 cm, even for scenes at close range.